This invention relates to coherent mixing of optical signals. Coherent mixing for detection finds applications not only in certain forms of information transmission systems, but also in certain forms of sensor systems. One of the problems of a coherent system is that detection efficiency falls to zero if ever the two signals being mixed are allowed to come into phase quadrature or their polarisation states are allowed to become orthogonal. In some coherent systems phase may present no particular problem, while in others polarisation may be no problem. Thus phase is no problem in a heterodyne system because the relative phase is continually changing, executing complete cycles at the beat frequency. Similarly polarisation is no problem in a system where states of polarisation are well defined throughout the system, for instance by the use of polarisation maintaining optical fibre. Where, however, phase or polarisation is a potential problem, a solution to that problem may be found by the use of diversity techniques.
One way of providing phase diversity is to make use of a 3=3 fibre coupler as described in a paper by A. W. Davis and S. Wright entitled "A phase insensitive homodyne optical receiver" given at the IEE Colloquium on Advances in Coherent Optical Devices and Technologies, 26 March, 1985, and in Specification No. GB 2 172 766A. It would be preferable to have outputs in phase quadrature instead of 120.degree. apart in phase, and a network providing this, which employs four 2=2 couplers, has been described by D. W. Stowe and T. Y. Hsu in a paper entitled "Demodulation of Interferometric Sensors Using a Fiber-Optic Passive Quadrature Demodulator" IEEE/OSA Journal of Lightwave Technology LT-1, No. 3 pp. 519-523 (Sept. 1983). In this network the four couplers are connected in two pairs, with the outputs of the first pair feeding the inputs to the second pair. A drawback of this approach is that a specific phase relationship is required for the inputs to the second pair of couplers for the network to function properly, and this means that the stability and relative optical path length of the connections between the first and second pairs of couplers must be maintained with great precision.